Currently, lithium ion secondary batteries have been commercialized as non-aqueous electrolyte secondary batteries for portable devices such as mobile phones. As weight and thickness of the portable devices are reduced, the non-aqueous electrolyte lithium ion secondary batteries themselves need to be thinner. Recently, thin batteries each using a laminate film as a sheath material are developed, and a laminate-type thin batteries are being put into practical use in which a positive electrode active material is a lithium cobalt oxide (LiCoO2), a negative electrode active material is a graphite material or carbon material, and a non-aqueous electrolyte is a lithium salt dissolved in an organic solvent or a polymer electrolyte.
Moreover, along with an increase in functions and enhancement of performance of the portable devices in recent years, power consumption of the devices is increasing. Batteries as power supplies thereof have been strongly required to increase capacities. Accordingly, Li—Ni oxide (LiNiO2, LixNi1-a-bCOaAlbO2), which can be expected to have higher capacity than conventional lithium-cobalt oxide, is being developed.
In recent years, aside from such an application, to promote introduction of electric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cell vehicles (FCVs) against a backdrop of rising environmental movement, power supplies for motor drive applications, auxiliary hybrid power supplies, and the like are being developed. For such applications, the non-aqueous electrolyte lithium ion secondary batteries, which can be repeatedly charged and discharged, are used. For applications which require high output and high energy density like motor drive applications for EVs, HEVs, FCVs, and the like, a single large battery cannot be fabricated practically, and an assembled battery composed of a plurality of batteries connected in series is generally used. As a battery constituting such an assembled battery, it has been proposed to use a laminate-type thin non-aqueous electrolyte lithium ion battery (just referred to as a thin laminate battery).
In a thin laminate battery in the applications requiring high output and high energy density, a metallic sheet material is used as a sheath member of the battery. The sheath member of this thin laminate battery is rectangular when viewed from the top and has a predetermined flat shape.
The thin laminate battery is lightweight because the thin laminate battery individually does not have a container made of metal. Moreover, when high voltage inside the container due to overcharge or the like causes rapture, shock is smaller than that in the metallic container. Accordingly, the thin laminate battery is suitable for applications which require high output and high energy density such as motor drive applications for EVs, HEVs, and FCVs.
Furthermore, in such thin laminate batteries, the requirement for increased capacity is further strengthened like the case of the aforementioned portable devices. Accordingly, the Li—Ni oxide, which can be expected to have higher capacity than the conventional lithium cobalt oxide, is being developed.
However, the Li—Ni battery using a positive electrode material including this Li—Ni oxide as the positive electrode active material (just referred to as a Li—Ni positive electrode material) has a problem that oxygen ions are oxidized by nickel ions with high valence within the positive electrode material into oxygen radicals and released to decompose an electrolysis solution. A large amount of gas is generated in the battery using the positive electrode material when the battery is initially charged or stored at high temperature, and the battery greatly swells.
To solve the aforementioned problem, the Japanese Patent Application Laid-Open No. 2002-203552 discloses a method of suppressing the gas formation by controlling pH of the positive electrode material.